THE STRUCTURE AND AFFINITIES OF DIPLOLAJBIS ROMERI (SOLMS). 721 



This xylem consists of a short rectangular central portion — the horizontal bar of the 

 H — , # and four arms, one arising from each angle. These arms are thinnest where 

 they join on to the horizontal bar of xylem, and they become considerably broader 

 near their free ends. At their extremities, however, each arm tapers sharply and ends 

 in an acute claw-like portion which is bent almost at right angles to the arm. The 

 smallest elements are found in the sinus formed between the arm and the claw-like 

 end. These claw-like ends are the beginnings of a pair of pinna-traces. They are 

 not present just after the pinna-trace-bar (such as b, PL IV. fig. 42) has departed from 

 the petiole-stele. At the departure of the pinna-trace-bar the ends of the arms are 

 blunt (PL II. fig. 29, and PL IV. fig. 42). In the latter figure— fig. 42— the blunt ends 

 are indicated at a, a, but the bar b is only completely detached from the arms two 

 sections higher up the petiole. The arms face one another in pairs, and so each end 

 of the trace resembles a pair of engineers' calipers, more or less closed, according as 

 the pinna-traces are completely developed or not. In the figure referred to above 

 (fig. 42), the space between one pair of arms is completely bridged over by the pinna- 

 trace-bar (6), which at a later stage breaks away and divides to supply a pair of pinnse. 

 The pair of arms on the opposite side of the trace are also developing xylem which 

 will ultimately form a complete bridge between the arms. In its turn, this bridge 

 will leave the petiole-trace and give rise to the next pair of pinna-traces. This side 

 of the petiole-trace shows remarkably well that each arm contributes equally to form 

 the bridge (fig. 42, c, c). In PL II. fig. 29, another specimen is shown where there are 

 no bridges between the arms, but the main trace is exactly similar to that of PL IV. 

 fig. 42. This figure (29) represents a section at the outer end of a series (No. 3, 

 text-fig. 1) which we shall follow in towards the stem. It is therefore a very important 

 illustration, since the whole interpretation of the specific character of this plant depends 

 on the identity of the petiole of fig. 42 with that of fig. 29. In another series, similar 

 to No. 3, text-fig. 1 (viz. No. 2, text-fig. 1) the last stage shown is just slightly beyond 

 that figured in fig. 29. The chief difference in this series is that a pair of reduced 

 pinnae are represented by their traces only. These traces are very small, but can 

 easily be distinguished. They can be followed into the petiole-trace ; indeed, fig. 28 

 (from Series 2) is the section where the last appearance of these reduced pinnae can be 

 detected. The two left-hand arms in that figure are connected by a bar of xylem 

 elements, though it can only be followed in the photograph for a short distance from 

 the upper left-hand arm. The bar is merely a thread one cell deep, but it is complete. 



Returning now to fig. 29 and passing down the petiole, the first change to be noted 

 is that the arms diminish in breadth near their outer ends, and remain either equally 

 broad until they taper at their extremities (PL II. fig. 28, d and 27) ; or they diminish 

 gradually throughout their length, and then suddenly taper as before at the ends 

 (PL II. fig. 26). 



* The horizontal bar is equivalent to the "apolaire mediane" of Bertrand. (Vide Etudes sur la fronde des 

 Zygopteridees.) 



